The major reason for the greater longevity of women is their relative protection, across all age groups, from ischemic heart disease (IHD). The traditional idea that estrogen protects women against IHD has recently been called into question. To investigate the potential contribution of X-chromosome gene(s) to the protection from IHD, we examined IHD risk factors in women with TS. TS is characterized by short stature, premature ovarian failure, cardiovascular anomalies and premature IHD. To control for the ovarian failure in TS, we compared glucose tolerance, lipid metabolism and blood pressure and in lean, young women with TS and age- and body composition-matched women with 46,XX premature ovarian failure (POF). Diabetes mellitus (DM) is a major cardiac risk factor. We have shown that while most girls and women with TS have normal fasting glucose and insulin, the glycemic response to a glucose challenge is dramatically abnormal and consistent with diabetes in about 40%, and is significantly above the POF control group in all women with TS. Interestingly, the glucose intolerance in these young lean women and girls with TS is not explained by insulin resistance, but by a novel insulin secretory defect. The insulin response to an oral or IV glucose challenge is significantly lower than POF or normal controls in all women with TS. It thus appears that the Turner ?metabolic syndrome? is not secondary to obesity or hypogonadism as previously thought. Rather it is a distinct entity characterized by decreased insulin secretion, reminiscent of mature onset diabetes of the young (MODY) syndromes, caused by haploinsufficiency for autosomal genes involved in pancreatic development-- suggesting that haploinsufficiency for unknown X-chromosome gene(s) impairs beta cell function and predisposes to DM in TS. We have also found LDL cholesterol and triglycerides are all significantly increased in TS compared to age- and BMI-matched women with POF. Moreover, NMR spectroscopy revealed a concentration of smaller, denser HDL and LDL lipid particles in women with TS. These data show a distinctly atherogenic lipid profile in otherwise healthy, non-obese young women with TS. Evidence from the study of women with monosomy X, or Turner syndrome (TS), suggests that dosage sensitive X-chromosome genes may contribute to normal women?s relative protection from ishemic heart disease (IHD) by suppressing atherogenic lipids, independent of gonadal effects. Normal young, 46,XX women also have lower average blood pressure (BP) values than men, suggesting that the 2nd X-chromosome might also contribute to BP modulation in women, and thus add to their protection from IHD. Confirming this hypothesis, we found that systolic and diastolic BPs were ~10% higher and heart rate 17% higher in women with TS compared to women with 46,XX POF (P<0.0001 for all 3 comparisons). Higher BP in TS could not be attributed to adiposity or congenital heart or renal defects. To evaluate parental imprinting as a source of asymmetry in X-gene dosage, we compared BP within the TS group after identifying the parental source of the subjects? single normal X chromosome. We found that the Xmat group had greater systolic (P=0.008), and diastolic BP (P=0.003) and heart rate (P=0.0005) compared to the Xpat group. There are at least two major mechanisms whereby a 2nd X chromosome in women could contribute to moderation of BP; certain X-chromosome genes involved in BP regulation may escape inactivation and thus normally be active in two copies in 46.XX women. Alternatively, or additionally, parental imprinting of X-chromosome genes involved in BP may have favorable effects in women. For example, a gene that exerts a moderating effect could be imprinted, or silenced on the maternal X (Xmat) but active from the paternal X allele. Since men only receive the Xmat, they would not experience the moderating effects on BP, but normal women with random X inactivation would express the Xpat allele in about 50% of their cells. Our novel findings implicating haploinsufficiency for X-chromosome genes in dyslipidemia, diabetes and high BP explain the increased risk for IHD in women with TS, and may also account for some of the increased risk for IHD among normal XY men compared to women. The identification of these genes clearly is of great clinical importance. The normal ovary produces abundant quantities of testosterone in addition to estradiol, but usual hormone ?replacement? treatment (HRT) for ovarian failure consists of estrogen and progesterone for most women with a uterus, or estrogen alone for smaller numbers of hysterectomized women. The risk of breast cancer is increased in menopausal women with such treatment, however, limiting its usefulness. We have previously shown that androgens have anti-mammogenic effects and inhibit estrogen?s mitogenic effects on the mammary epithelium. In some countries, including Australia, testosterone is often prescribed for menopausal women in addition to usual HRT. The rationale for testosterone supplementation has been that estrogen treatment reduces residual ovarian androgen production in post-menopausal women and may also lead to sequestration of available testosterone by increasing sex hormone binding globulin, resulting in symptoms of asthenia and loss of libido in some women. Therefore we undertook a systematic review of breast cancer incidence in an Australian clinic population where women are routinely treated with testosterone along with usual HRT. Breast cancer status was ascertained by mammography at the beginning of testosterone treatment and biannually thereafter with mean duration of follow-up equal to 5.8 ? 2.5 years. The mean age of the women at the start of observation was 56.4 yrs. Within this observation period, 7 invasive breast cancer cases were diagnosed among these women, resulting in an incidence of 239 per 100,000 women-years. Notably, 6/7 cases and the only death occurred in the estrogen/progestin group. These rates are substantially lower than those reported for age-matched women receiving conventional hormone treatment. For example, the Women?s Health Initiative study reports a rate of 380 per 100,000 women-years in women receiving estrogen plus progestin and the ?Million Woman? Study reported 430 cases per 100,000 woman years for current HRT users compared with 283/100,000 for never users. The prevalence of a positive family history was rather high in our group, suggesting a higher risk for breast cancer at baseline, making the present observations of breast cancer rates similar to untreated post-menopausal women all the more remarkable. These observations suggest that the addition of testosterone to conventional HRT for post-menopausal women does not increase, and may indeed reduce the HRT-induced breast cancer risk, returning thereby the incidence to normal rates of the order of those observed in the general untreated population. Follow-up studies using Genechip and protein arrays to detect molecular effects of testosterone upon the primate mammary epithelium.